Fixed the quad mask in a very specific case and changed saveVTK to write a
specific value for unused nodes instead of the first one which breaks unit
tests depending on the number of ranks used.
Still need to rebaseline some VTK files so the tests pass.

Reincarnation of the escriptreader as a more flexible escriptexport library:
- can be initialised with instances of escript::Data and Finley_Mesh
- is now MPI aware at the EscriptDataset level including Silo writer
- now uses boost shared pointers
The lib is currently only used by the escriptconvert tool but this is going to
change...